(1) Field of the Invention
This invention relates to mobile phones and in particular it relates to a security function for restricting the use of individual radio telephone hand sets to selected operator networks.
The hand set for which the invention is provided is a digital radio transceiver programmed to operate under the control of a central processing unit (CPU).
(2) Description of the Prior Art
Owing to the variations in pricing policy of network operators, the nominal selling price of mobile phones is often heavily discounted in favour of a higher call tariff. The possibility exists therefore of a user obtaining at very low, or even no cost, a hand set for use on a specified network with a relatively high call tariff. If the hand set were then to be used on a low call tariff network for which the charge for hand sets remained at full selling price then both network operators would be deprived of revenue. It is important to network operators therefore that use of hand sets be restricted to specified networks.
The adaptation of telephone hand sets by copying from or imitating a licensed hand set, normally for unauthorized sale or use, is known as cloning and this definition of cloning is used herein.
Prior art methods for prevention of unauthorized use and prevention of cloning of telephone hand sets include so called secret numbers and U.S. Pat. No. 5,430,892 for example describes a use of secret numbers stored in a ROM (read only memory).
Removal of the ROM from the circuit board of a telephone hand set properly connected to a target network would, however, allow unauthorized copies of the ROM to be obtained. An unscrupulous person would then be in a position to alter very many telephone hand sets merely by supplying the appropriate ROM.